Tunable receivers used in cable television set-top boxes and in other applications receive broadband signals having many channels. It is a function of such a tunable receiver to produce the signals in a desired channel and reject the signals in the remaining channels. In rejecting the signals in the undesired channels, the tunable receiver should substantially remove all signals associated with the undesired channels so that the receiver outputs substantially the signals in the desired channel. These unwanted signals include image, harmonics, spurious, and other undesired signals.
Both single conversion and double conversion receivers are known in the prior art. In a single conversion receiver, the RF signal is directly down converted to the desired frequency range, either to an intermediate frequency (IF) or sometimes to baseband. In a double conversion receiver, two frequency conversions are used. Usually the selected channel is up shifted in frequency to a fixed frequency, and then down shifted from that frequency to a fixed IF frequency. In either case, various circuits with differing degrees of integration are known for the purpose.